P-type isolation between QCL regions

ABSTRACT

A quantum cascade laser and its method of fabrication are provided. The quantum cascade laser comprises one or more p-type electrical isolation regions and a plurality of electrically isolated laser sections extending along a waveguide axis of the laser. An active waveguide core is sandwiched between upper and lower n-type cladding layers and the active core and the upper and lower n-type cladding layers extend through the electrically isolated laser sections of the quantum cascade laser. A portion of the upper n-type cladding layer comprises sufficient p-type dopant to have become p-type and to have become an electrical isolation region, which extends across at least a part of the thickness upper n-type cladding layer along a projection separating the sections of the quantum cascade laser.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.13/050,058, filed Mar. 17, 2011, for “P-Type Isolation Regions AdjacentSemiconductor Laser Facets.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

A quantum cascade laser (QCL) is a unipolar semiconductor device thatcan be readily engineered to emit over a variety of wavelengths,including but not limited to, the mid-infrared and terahertz portions ofthe electromagnetic spectrum. Device growth and processing can be basedon established techniques and widely available materials, such as InPand GaAs and other III-V semiconductor materials. The present disclosurerelates to quantum cascade semiconductor lasers (QCLs) and, moreparticularly, to methods of fabricating QCLs and the corresponding QCLstructures.

2. Technical Background

The present inventors have recognized that, in semiconductor lasers thatutilize interband lasing transitions, the quantum wells and barrierlayers that compose the active region are often sandwiched betweenn-type and p-type layers on opposite sides of the active region, withthe p-type layers typically above the active region. These p-type layersare typically not very conductive. Therefore, interrupting theelectrical contact layer or metal on top of the p-doped layer(s)typically provides sufficient electrical isolation between distinctregions of the laser structure. In contrast, the present inventors haverecognized that a QCL is a unipolar device where the layers both belowand above the active core are of the same conductivity type, typicallyn-type, and that n-type layers are highly conductive. Accordingly,electron diffusion from one area to an adjacent area above the activecore cannot be prevented simply by interrupting the electrical contactlayer or metal between sections of the n-type layer to be electricallyisolated.

Although the methodology of the present disclosure has applicability toa variety of semiconductor laser configurations, the present inventorshave recognized that the need for effective isolation is particularlyacute in the context of distributed Bragg reflector (DBR) QCLs, whichcomprise an active region, a wavelength selective region, and,optionally, a phase region.

BRIEF SUMMARY

In accordance with one embodiment of the present disclosure, a quantumcascade laser and its method of fabrication are provided. The quantumcascade laser comprises one or more p-type electrical isolation regionsand a plurality of electrically isolated laser sections extending alonga waveguide axis of the laser. An active waveguide core is sandwichedbetween upper and lower n-type cladding layers. The active core and thelower n-type cladding layer, as well as at least part of the uppercladding layer, extend through the electrically isolated laser sectionsof the quantum cascade laser. A portion or portions of the upper n-typecladding layer comprise sufficient p-type dopant to define p-typeelectrical isolation region(s), which extend across part of thethickness of the upper n-type cladding layer along a projectionseparating the sections of the quantum cascade laser. The upper andlower n-type cladding layers may comprise InP, GaAs, AlGaAs, or anyother conventional or yet-to-be developed cladding material suitable forthe fabrication of a QCL. For example, and not by way of limitation, itis contemplated that a variety of cladding materials might be suitablein a QCL, including II-VI semiconductors, Si—Ge or GaN-based materials,etc.

In accordance with other embodiments of the present disclosure, laserstructures are also contemplated where isolation regions are solelyprovided adjacent one or both of the laser facets to provide verticalisolation, reduce current, and help minimize potentially harmful facetheating.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a schematic illustration of a DBR quantum cascade lasercomprising an active gain section, a wavelength selective section, and awindow section;

FIG. 2A is a longitudinal schematic illustration of a DBR quantumcascade laser comprising p-type electrical isolation regions;

FIG. 2B illustrates an alternative to the window configurationillustrated in FIG. 2A;

FIG. 3 is a transverse schematic illustration of a DBR quantum cascadelaser comprising p-type electrical isolation regions;

FIGS. 4A and 5A are longitudinal schematic illustrations of DBR quantumcascade lasers according to two alternative embodiments of the presentdisclosure;

FIGS. 4B and 5B illustrate alternatives to the window configurationsillustrated in FIGS. 4A and 5A; and

FIGS. 6 and 7 are schematic illustrations of laser structures whereisolation regions are solely provided adjacent the facets of asemiconductor laser.

DETAILED DESCRIPTION

Although the concepts of the present disclosure enjoy applicability toany type of multi-section QCL, specific embodiments of the presentdisclosure are illustrated herein with reference to DBR quantum cascadelasers. Nevertheless, the present disclosure and accompanying claimsshould not be limited to DBR lasers or to the specific materialsmentioned in the present description, unless otherwise expressly noted.For example, and not by way of limitation, FIG. 1 is a schematicillustration of a DBR quantum cascade laser comprising an active gainsection 10, a wavelength selective section 12 commonly referred to as aDBR section, and an output window section 14. As will be appreciated bythose familiar with DBR quantum cascade lasers, the active gain section10 of the DBR quantum cascade laser provides the major optical gain ofthe laser while the wavelength selective section 12 provides forwavelength selection. For example, although the wavelength selectivesection 12 may be provided in a number of suitable configurations thatmay or may not employ a Bragg grating, in many cases the wavelengthselective section 12 comprises a first order or second order Bragggrating that is positioned outside the active gain section 10 of thelaser cavity. The grating acts as a mirror whose reflection coefficientdepends on wavelength.

FIGS. 4A, 4B, 5A and 5B illustrate three section DBR lasers where aphase section 16 is provided between the wavelength selective section 12and the active gain section 10 of the DBR quantum cascade laser. Thephase section 16 creates an adjustable phase shift between the gainsection 10 and the wavelength selective section 12. The phase section 16may also be used to reduce the thermal coupling between the gain section10 and the wavelength selective section 12 to reduce the lasingwavelength shift due to thermal cross-talk. The concepts of the presentdisclosure enjoy applicability to all types of DBR quantum cascadelasers, regardless of whether they are two-section, three-section, orfour-section DBR lasers.

The DBR quantum cascade lasers illustrated in FIGS. 1 and 2A eachcomprise an active waveguide core 20 sandwiched between upper n-typecladding layers 22, 26 and a lower n-type cladding layer 24. Theadditional upper n-type cladding layer 26 is a highly n-type dopedlayer, compared to the n-type cladding layer 22 which is a relativelylow n-type doped layer. The active core 20 and the upper and lowern-type cladding layers extend through the active gain section 10 andthrough the wavelength selective section 12 of the DBR quantum cascadelaser. In a quantum cascade laser (QCL), the core layer 20 comprisesalternating semiconductor layers that are configured to amplify lightemitted during carrier transitions between energy states within the sameenergy band. A QCL is often referred to as a unipolar device because itutilizes quantum well transitions of a single carrier type. Most QCLsuse electron transitions, in which case the layers both below and abovethe core are n-type cladding layers. The active gain and wavelengthselective sections 10, 12 are capped with a patterned electrical contactlayer 30 which comprises respective control electrodes dedicated to thedifferent sections 10, 12 of the laser. An insulating dielectricmaterial 32 is deposited in appropriate regions in the patternedelectrical contact layer 30 to isolate electrically the distinct regionsof the laser structure. Nevertheless, the present inventors haverecognized that the DBR quantum cascade laser is subject to substantialelectron diffusion from a non-dielectric-capped area to adjacentdielectric-separated areas.

In FIG. 2A, portions of the upper n-type cladding layers 22, 26 areprovided with sufficient p-type dopant to define one or more p-typeelectrical isolation regions 40. Preferably, these electrical isolationregions 40 extend across a part of the thickness of the upper n-typecladding layers 22, 26 along respective projections separating theactive gain section 10, the wavelength selective section 12, and theoutput window section 14 of the DBR quantum cascade laser. It iscontemplated that these electrical isolation regions 40 can extendacross a part or, more specifically, a majority of the thickness of theupper n-type cladding layers 22, 26. As is illustrated in FIG. 2B, it iscontemplated that the window section 14 can be all p-doped above theactive core 20. It is further contemplated that a corresponding windowsection can be provided at the input facet of the laser structure.Additionally, it is contemplated that the window sections need not beprovided in the laser structure at all.

There are diverse ways of realizing the p-type isolation regions. Amongthese are selective growth, ion implantation, and diffusion of a p-typedopant. If the last option is chosen, the respective compositions of theupper and lower n-type cladding layers 22, 24, 26 and the activewaveguide core 20 may be selected to facilitate formation of the p-typeelectrical isolation regions 40 by dopant diffusion. More specifically,the upper and lower n-type cladding layers 22, 24, 26 may comprise InPand the p-type dopant may be selected such that its maximum stableconcentration in the InP upper n-type cladding layer is belowapproximately n×10¹⁸ cm⁻³, where n is less than 3.

By way of example, and not limitation, it is alternatively contemplatedthat the upper and lower n-type cladding layers 22, 24, 26 may beGaAs-based cladding layers. Some of the cladding layers may be AlGaAs or(Al)GaInP instead of simply GaAs or InP. For GaAs-based cladding layers,the core may be GaAs/AlGaAs, AlGaAs/AlGaAs, (Al)GaInP/(Al)GaInP, orGaInAs/(Al)GaAs. Additional layers of similar composition arecontemplated for the remaining layers of the structure and should beselected to compensate for any lattice-mismatch between GaInAs and theGaAs substrate. For example, and not by way of limitation, otherpossible layers are GaInP, AlGaInP, GaAsP, and GaInAsP. For GaAs-basedcladding layers, suitable dopants used to make (Al)GaAs semi-insulatinginclude, but are not limited to Cr and O. At very low temperaturegrowth, semi-insulating (Al)GaAs can be obtained without any dopant.

In some embodiments, the alternating semiconductor layers of the activewaveguide core 20 comprise alternating Group III-V materials in whichthe maximum stable concentration of the dopant in the core is at least afactor of 10 greater than the maximum stable concentration of the p-typedopant in the upper n-type cladding layer. In some instances, theresulting device, which comprises core and cladding layers that definesubstantially different maximum stable dopant concentrations can be wellsuited for formation of the p-type electrical isolation regions 40 ofthe present disclosure, particularly where there is a desire to keep thedopant out of the core. In other instances, it may be preferable toallow the dopant to diffuse into the core 20.

The p-type electrical isolation regions 40 can also be formed by ionimplantation, in which case, it will merely be necessary to ensure thatthe p-type dopant defines a p-doping concentration that is higher thanthe n-doping concentration of the upper cladding layer.

More specifically, the QCL illustrated in FIG. 2A is grown on an InPsubstrate 50 and the core 20 is surrounded by InP cladding layers 22,24, 26. The maximum-stable concentration of p-type dopants like Zn, Cd,Be, Mg, and Mn are relatively low in InP. A p-type dopant diffusesrelatively quickly above its maximum-stable concentration and itsdiffusion coefficient increases super-linearly with its concentration.For example, the maximum-stable concentration of Zn in InP is betweenapproximately 1×10¹⁸ cm⁻³ and approximately 2×10¹⁸ cm⁻³. Accordingly,when diffusion of Zn is used to form the p-type electrical isolationregions 40 of the present disclosure, the Zn-dopant concentration in thearea converted to p-type via diffusion will be no more thanapproximately 2×10¹⁸ cm⁻³. The present inventors have recognized that alow dopant level is advantageous because optical loss increases withcarrier concentration, so not having to introduce a high concentrationof p-type dopant keeps the loss low. Further, at this concentrationlevel, Zn diffuses little through the QCL core 20, which may, forexample, be composed of GaInAs and AlInAs, partly because theconcentration of the diffusing Zn is a factor of 10 lower than themaximum stable concentration of the dopant in the core 20. For example,and not by way of limitation, the maximum stable concentration oftypical dopants in a core composed of GaInAs and AlInAs is betweenapproximately 1×10¹⁹ cm⁻³ and approximately 6×10¹⁹ cm⁻³. Accordingly, ifZn diffuses down to the core 20, it will be stopped very quickly insidethe top layers of the core 20. Typically the p-type electrical isolationregions 40 extend from the top of the core to within approximately 1.5μm of the core layer 20.

Although QCLs according to the present disclosure can be fabricatedusing a variety of materials and layer configurations, it iscontemplated that the portion of cladding layer 22 through which Zn isto be diffused be between 0.5 and 3 μm, with an original n-type dopingdensity of less than 0.5×10¹⁸ cm⁻³. Similarly, the core 20 may comprisealternating layers of Group III-V wells and Group III-V barriers and,more specifically, alternating layers of GaInAs wells and AlInAsbarriers or Sb-containing material(s). The core region typicallyincludes a plurality (e.g., 10 or more, but typically more than 20) ofessentially identical multilayer semiconductor repeat units selected forlasing in a wavelength range centered around the particular desiredlasing wavelength.

Referring now to FIGS. 4A and 5A, it is noted that the p-type electricalisolation regions 40 according to the present disclosure may extendacross the upper n-type cladding layer 22 along projections separatingthe active gain section 10, the wavelength selective section 12, theoutput window section 14, and the phase section 16 of the DBR quantumcascade laser. Although the wavelength selective, phase, and windowsections of the QCL are illustrated in FIGS. 4A and 5A with dedicatedpatterned portions of an electrical contact layer 30, it is contemplatedthat these sections may be active or passive. As is illustrated in FIG.4B, it is contemplated that the window section 14 can be all p-dopedabove the active core 20. FIG. 5B illustrates an alternative to thewindow configuration illustrated in FIG. 5A where the trench of FIG. 5Ais not utilized, the window section 14 is partially p-doped above theactive core 20, and the metal contact is positioned over the top of thepartially p-doped section for better heat sink.

In many embodiments, it is contemplated that the window sections 14described herein will not be provided with a p-type electrical isolationregion, particularly where there is no desire to provide verticalisolation in the window sections of the laser. Conversely, laserstructures are also contemplated where isolation regions are solelyprovided in one or both of the window sections of the semiconductorlaser to provide vertical isolation therein. More specifically,referring to FIGS. 6 and 7, as will be appreciated by those practicingthe concepts of the present disclosure, semiconductor lasers willtypically comprise opposing facets 13, 14 that are configured withreflectivity characteristics suitable to form the resonant cavity of thelaser, with facet 13 forming the output window of the laser. The presentinventors have recognized a continuing drive to provide robust laserfacets in semiconductor lasers, particularly semiconductor lasers thatare subject to acute facet heating and substantial current injectioninto the active region of a laser in the vicinity of the laser outputwindow. These problems are particularly acute in the context of laserscharacterized by relatively high output powers and the presentdisclosure addresses the continuing drive to fabricate robust facets fora variety of semiconductor lasers including, but not limited to, DBR ornon-DBR quantum cascade laser, Fabry-Perot lasers, or any semiconductorlaser with input/output facets where the aforementioned degradation is aconcern.

To this end, it is contemplated that the upper n-type cladding layer 22can be provided with sufficient p-type dopant to define p-type isolationregions 40 adjacent at least one or each of the opposing facets 13, 14.It is contemplated that this isolation region 40 can extend across apart or, more specifically, a majority of the thickness of the uppern-type cladding layer 22 adjacent the facet 13 defining the laser outputwindow, the opposing facet 14, which may define the laser input window,or both facets 13, 14. The laser may be a DBR or non-DBR quantum cascadelaser, a Fabry-Perot laser, or any semiconductor laser with input and/oroutput facets. Constructed in this manner, it is contemplated that thep-type isolation region 40 will reduce the current into regions adjacentthe facets 13, 14 of the laser, and help minimize potentially harmfulfacet heating, without interfering with the structure of the activewaveguide core 20. Further, it is contemplated that, where the lasercomprises a patterned electrical contact layer 30 configured to initiateelectrical current injection into the active waveguide core 20, thep-type isolation region 40 will, with or without the aid of a facetdielectric 32, inhibit current injection into the facet regions of theactive waveguide core 20.

It is also contemplated that the opposing facets can be provided withanti-reflective coatings, highly reflective coatings, or combinationsthereof.

The illustrations of the present disclosure also show an additionaln-type InP cladding layer 26 sandwiched between the patterned electricalcontact layer 30 and the upper n-type cladding layer 22. The additionaln-type InP cladding layer 26 is more highly doped than the upper n-typecladding layer 22, e.g., up to about 1×10¹⁹ cm⁻³. The relatively highlydoped n-type InP cladding layer 26 has a relatively low refractive indexat the operational wavelengths of interest, which prevents the mode fromextending further and being absorbed by the contact layers and metals inthe laser structure. As will be appreciated by those familiar with laserconstruction and operation, the electrical contact metal is used tofacilitate the flow of an electrical current through the laser. In FIGS.2A and 4A, the p-type electrical isolation regions 40 extend across theupper n-type cladding layer 22 and the additional n-type InP claddinglayer 26.

As illustrated on FIG. 5A, trenches 36 may be etched at least partiallyor entirely through the additional n-type InP cladding layer 26.Trenches 36 may be particularly advantageous when the p-type electricalisolation regions 40 are formed by diffusing a p-type dopant into thelayers 22 and 26. By etching a trench through the additional layer 26,the p-type isolation region 40 may be more quickly diffused with ap-type dopant, such as Zn, because the dopant does not have to diffusethrough the entire additional n-type cladding layer 26 in order to reachand diffuse into the upper n-type cladding layer 22. An additionaldielectric isolation layer 34 is optionally provided on the walls oftrenches 36 formed in the additional n-type InP cladding layer 26. Withthis construction, the p-type electrical isolation regions 40 do notextend across a significant portion of the additional n-type InPcladding layer 26. Metal or other conductive materials that could shortcircuit the isolation regions 40 should not be permitted in the trenches36. Alternatively, p-type isolation regions 40 may be formed in theupper n-type cladding layer 22, either by implantation or diffusion,prior to formation of the additional n-type cladding layer 26. Theadditional n-type cladding layer 26 may then be formed on the uppern-type cladding layer 22. The upper portion of the p-type isolationregions 40 may then be formed in the additional n-type cladding layer 26either by implantation or diffusion. Alternatively, rather than dopingthe regions in the additional n-type cladding layer 26 above the p-typeisolation regions 40 formed in the upper n-type cladding layer 22, theregions in the additional n-type cladding layer 26 above the p-typeisolation regions 40 in the upper n-type cladding layer 22 may be etchedaway forming trenches 36 in the additional n-type cladding layer 26 aspreviously described herein.

In each of the illustrated configurations, the DBR quantum cascade lasercomprises an additional electrical contact layer 35 on the substrate 50.It is contemplated that the substrate 50 may be n-doped InP or any of avariety of group III-V materials suitable for a DBR quantum cascadelaser. The lower n-type cladding layer 24 may be moderately n-doped InP.In addition, it is contemplated that the DBR quantum cascade laser maycomprise one or more additional intervening layers interposed betweenthe active waveguide core 20 and the upper and lower n-type claddinglayers 22, 24. For example, the DBR quantum cascade laser may comprise alayer of GaInAs that is lattice-matched to InP, in which a grating maybe defined. The GaInA layer, or layer of another composition, eitheronly above or only below the core, or both below and above the core, mayalso serve as waveguide layer. As a further example, the DBR quantumcascade laser may comprise a buffer layer interposed between the lowern-type cladding layer 24 and the InP substrate 50. A buffer layertypically is used in semiconductor material epitaxial growth to form asmooth and clean surface for the growth of a high quality devicestructure, as will be appreciated by those skilled in the art. Thebuffer layer may be constructed of InP,indium-gallium-arsenide-phosphide (InGaAsP),indium-aluminum-arsenide-phosphide (InAlAsP),indium-aluminum-gallium-phosphide (InAlGaAs), or other Group III-Vmaterials, or, in an alternative embodiment, it need not be present atall. If the buffer layer is InP, it is part of layer 24. If a QCL isgrown by molecular beam epitaxy (MBE), the lower and upper claddinglayers can be arsenide materials (GaInAs, AlInAs) because MBE cannottypically grow InP, in which case the cladding layers 22, 24, or layersbetween the cladding layers 22 and 24, will be arsenide materials.

Referring to FIG. 3, it is noted that the upper and lower InP n-typecladding layers 22, 24 and the active waveguide core 20 can beconfigured as a ridge waveguide bounded by semi-insulating InP regions28. Semi-insulating InP can be generally grown by Fe doping. The presentinventors have recognized, however, that pronounced interdiffusion of Feand the p-type dopant in the upper cladding layer can be problematic inachieving reliable semi-insulating characteristics in thesemi-insulating regions bounding the ridge waveguide structure.Accordingly, the present disclosure contemplates the fabrication of morestable semi-insulating InP regions by replacing Fe with a less diffusivedeep trap or by adding to Fe another deep trap element including, forexample, the transition metal Rh or Ru or Ti.

Area selective Zn diffusion can be performed by masking part of thewafer with a mask such as, for example, SiO₂, and exposing the wholewafer to Zn diffusion, after growth of the core and of the thickness ofupper cladding layer 22 through which the p-sections are to extend. Znonly penetrates into the exposed semiconductor layers and thereforediffuses only in areas non-protected by SiO2. One possible technique forp-dopant diffusion is to heat the wafer after deposition on its surfaceof a gel containing this p-dopant. Zn diffusion can also be performed ina sealed ampoule, for example in the presence of a material, such as,Zn_(x)AS_(y or) Zn_(x)P_(y)l.

In one example Zn diffusion is performed inside a metal-organic vaporphase epitaxy (MOVPE) reactor, the mask being silicon dioxide. Thefollowing fabrication sequence is contemplated according to themethodology of this example:

-   -   1. An InP lower cladding layer and a core composed of GaInAs        wells and AlInAs barriers are grown. Subsequently, a selected        thickness of n-type InP is grown, with the thickness and doping        density of the n-type InP layer selected between approximately        0.5 μm and approximately 1.5 μm and approximately 2-10×10¹⁶        cm⁻³, respectively.    -   2. A relatively thin layer of mask material, such as SiO₂,is        deposited and patterned to protect areas where no Zn diffusion        is desired. Patterning can be realized by photolithography of a        photoresist mask, followed by wet or dry etching of the SiO₂        layer where not protected by photoresist.    -   3. The wafer is placed back into the MOVPE reactor and Zn is        made to diffuse into the unmasked areas. Our preferred technique        is to cause Zn diffusion into InP from a top-grown heavily Zn        doped GaInAs layer. The Zn concentration in GaInAs is higher        than its maximum stable concentration to enable the Zn to        diffuse into the n-type InP layer. At the interface between the        highly Zn doped GaInAs and the InP layer, the Zn concentration        will be higher than its stable concentration in InP, driving        diffusion.    -   4. The Zn doped GaInAs surface layers as well as the SiO₂ mask        can be etched away using dilute HF to dissolve the SiO2 and (10        H2O:1H2SO4:1H2O2) or any other suitable etchant for GaInAs.    -   5. A buried ridge QCL can be realized using any conventional or        yet-to-be developed growth methodology including, for example,        one of two ordinary options:        -   i—grow the upper n⁻/n⁺ layers over the whole wafer, then            define stripes, etch ridges downward through the core, and            regrow semi-insulating on the sides of the ridges or simply            coat the sides with an insulating material (so-called buried            ridge laser or ridge laser, respectively);        -   ii—grow only part of the upper n⁻/n⁺ layers, define stripes            and form ridges, deposit semi-insulating InP on the sides of            the ridges, remove the SiO₂ from the top of the ridges and            grow the top n⁻/n⁺ layers everywhere (so-called planar            buried heterostructure laser, the structure which we            adopted).    -   6. For lateral electrical isolation between two electrodes, the        n-doped layers above the p-doped layer in the short isolating        regions are preferably etched away before deposition of the        dielectric film.

It is noted that recitations herein of “at least one” component,element, etc., should not be used to create an inference that thealternative use of the articles “a” or “an” should be limited to asingle component, element, etc.

It is noted that recitations herein of a component of the presentdisclosure being “configured” in a particular way or to embody aparticular property, or function in a particular manner, are structuralrecitations, as opposed to recitations of intended use. Morespecifically, the references herein to the manner in which a componentis “configured” denotes an existing physical condition of the componentand, as such, is to be taken as a definite recitation of the structuralcharacteristics of the component.

For the purposes of describing and defining the present invention it isnoted that the terms “substantially” and “approximately” are utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation.

It is noted that terms like “preferably,” “commonly,” and “typically,”when utilized herein, are not utilized to limit the scope of the claimedinvention or to imply that certain features are critical, essential, oreven important to the structure or function of the claimed invention.Rather, these terms are merely intended to identify particular aspectsof an embodiment of the present disclosure or to emphasize alternativeor additional features that may or may not be utilized in a particularembodiment of the present disclosure.

Having described the subject matter of the present disclosure in detailand by reference to specific embodiments thereof, it is noted that thevarious details disclosed herein should not be taken to imply that thesedetails relate to elements that are essential components of the variousembodiments described herein, even in cases where a particular elementis illustrated in each of the drawings that accompany the presentdescription. Rather, the claims appended hereto should be taken as thesole representation of the breadth of the present disclosure and thecorresponding scope of the various inventions described herein. Further,it will be apparent that modifications and variations are possiblewithout departing from the scope of the invention defined in theappended claims. More specifically, although some aspects of the presentdisclosure are identified herein as preferred or particularlyadvantageous, it is contemplated that the present disclosure is notnecessarily limited to these aspects. For example, the illustrations ofthe present disclosure show laser diodes where current is injected intothe wavelength selective and phase sections of a DBR laser forwavelength control. As will be appreciated by those familiar with laserdiode construction and operation, heaters or heating elements can alsobe used to control these sections of a laser diode, in which case adielectric film would be positioned on the top of wavelength selectiveand phase sections of the laser diode below a metal heating element.

It is noted that one or more of the following claims utilize the term“wherein” as a transitional phrase. For the purposes of defining thepresent invention, it is noted that this term is introduced in theclaims as an open-ended transitional phrase that is used to introduce arecitation of a series of characteristics of the structure and should beinterpreted in like manner as the more commonly used open-ended preambleterm “comprising.”

What is claimed is:
 1. A quantum cascade laser comprising one or morep-type electrical isolation regions and a plurality of electricallyisolated laser sections extending along a waveguide axis of the laser,wherein: the quantum cascade laser comprises an active waveguide coresandwiched between upper and lower n-type cladding layers; the activecore and the upper and lower n-type cladding layers extend through theelectrically isolated laser sections of the quantum cascade laser; aportion of the upper n-type cladding layer comprises sufficient p-typedopant to define a p-type electrical isolation region that extendsacross at least a part of the thickness of the upper n-type claddinglayer along a projection separating the electrically isolated lasersections of the quantum cascade laser; and the core layer comprisesalternating semiconductor layers that are configured to amplify lightemitted due to carrier transitions between energy states within the sameenergy band.
 2. A quantum cascade laser as claimed in claim 1 whereinthe upper and lower n-type cladding layers comprise InP and the p-typedopant is selected such that its maximum stable concentration in theupper cladding layer is below approximately n ×10¹⁸ cm⁻³, where n isless than
 3. 3. A quantum cascade laser as claimed in claim 1 wherein:the quantum cascade laser comprises a patterned electrical contact layerconfigured to separately initiate electrical current injection intosections of the active waveguide core; and the p-type electricalisolation region is configured for lateral electrical isolation betweenthe electrically isolated laser sections of the active waveguide core.4. A quantum cascade laser as claimed in claim 1 wherein the p-typeelectrical isolation region is formed by dopant diffusion into andthrough the portion of the upper n-type cladding layer.
 5. A quantumcascade laser as claimed in claim 4 wherein the portion of the uppern-type cladding layer through which the dopant is diffused defines alayer thickness of between approximately 0.5 μm and approximately 2.5μm.
 6. A quantum cascade laser as claimed in claim 4 wherein the portionof the upper n-type cladding layer through which the dopant is diffuseddefines an n-type doping density of up to approximately 5 ×10¹⁷ cm⁻³. 7.A quantum cascade laser as claimed in claim 1 wherein the p-typeelectrical isolation region extends to within approximately 1.5 μm fromthe core layer.
 8. A quantum cascade laser as claimed in claim 1 whereinthe concentration of the p-type dopant in the upper n-type claddinglayer is no more than approximately 2 ×10¹⁸ cm⁻³.
 9. A quantum cascadelaser as claimed in claim 1 wherein the alternating semiconductor layerscomprise alternating Group III-V materials selected such that themaximum stable concentration of the dopant in the core is at least afactor of 10 greater than the maximum stable concentration of the p-typedopant in the upper n-type cladding layer.
 10. A quantum cascade laseras claimed in claim 1 wherein the core comprises alternating layers ofGroup III-V wells and Group III-V barriers.
 11. A quantum cascade laseras claimed in claim 1 wherein the core comprises alternating layers ofGaInAs wells and AlInAs barriers.
 12. A quantum cascade laser as claimedin claim 1 wherein the quantum cascade laser comprises a DBR laser andthe plurality of electrically isolated laser sections of the quantumcascade laser comprise an active gain section and a wavelength selectivesection.
 13. A DBR quantum cascade laser as claimed in claim 12 wherein:the DBR quantum cascade laser further comprises a phase section; theactive core and the lower n-type cladding layer and most of the uppercladding layer extend through the active gain section, the phasesection, and the wavelength selective section of the DBR quantum cascadelaser; the p-type electrical isolation region extends across at least apart of the thickness of the upper n-type cladding layer along aprojection separating the active gain section from the phase section andthe wavelength selective section of the DBR quantum cascade laser; andan additional portion of the upper n-type cladding layer comprisessufficient p-type dopant to define an additional p-type electricalisolation region extending across at least a part of the thickness ofthe upper n-type cladding layer along a projection separating the phasesection and the wavelength selective section of the DBR quantum cascadelaser.
 14. A DBR quantum cascade laser as claimed in claim 12 wherein:the quantum cascade laser further comprises an output window section;the active core and the upper and lower n-type cladding layers extendthrough the active gain section, the output window section, and thewavelength selective section of the quantum cascade laser; and anadditional portion of the upper n-type cladding layer comprisessufficient p-type dopant to define an additional p-type electricalisolation region extending across at least a part of the thickness ofthe upper n-type cladding layer along a projection separating the gainsection and the output window section of the quantum cascade laser. 15.A DBR quantum cascade laser as claimed in claim 12 wherein: the DBRquantum cascade laser comprises a patterned electrical contact layer;the patterned electrical contact layer comprises respective controlelectrodes dedicated to the wavelength selective section and the gainsection, respectively; and the p-type electrical isolation regionextends along a projection separating the active gain section and thewavelength selective section of the DBR quantum cascade laser.
 16. Aquantum cascade laser as claimed in claim 1 wherein the upper and lowern-type cladding layers and the active waveguide core are configured as aridge waveguide and lateral confinement in the quantum cascade laser isprovided by a raised ridge or semi-insulating regions bounding the ridgewaveguide.
 17. A DBR quantum cascade laser as claimed in claim 1wherein: the p-type electrical isolation region is formed by dopantdiffusion; the maximum stable concentration of the p-type dopant in theupper n-type cladding layer is no more than approximately 2 ×10¹⁸ cm ⁻³;and the upper and lower n-type cladding layers and the active waveguidecore are configured as a ridge waveguide bounded by semi-insulatingregions.
 18. A quantum cascade laser comprising one or more p-typeelectrical isolation regions and a plurality of electrically isolatedlaser sections extending along a waveguide axis of the laser, wherein:the quantum cascade laser comprises an active waveguide core sandwichedbetween upper and lower n-type cladding layers; the active core and theupper and lower n-type cladding layers extend through the electricallyisolated laser sections of the quantum cascade laser; a portion of theupper n-type cladding layer comprises sufficient p-type dopant to definea p-type electrical isolation region that extends across at least a partof the thickness of the upper n-type cladding layer along a projectionseparating the electrically isolated laser sections of the quantumcascade laser; the p-type dopant defines a p-doping concentration thatis higher than the n-doping concentration of the upper cladding layer;and the core layer comprises alternating semiconductor layers that areconfigured to amplify light emitted due to carrier transitions betweenenergy states within the same energy band.
 19. A method of fabricatingone or more p-type electrical isolation regions in a quantum cascadelaser, comprising: providing a quantum cascade laser that comprises aplurality of electrically isolated laser sections and an activewaveguide core sandwiched between upper and lower n-type claddinglayers, with the active core and the upper and lower n-type claddinglayers extending through the electrically isolated laser sections of thequantum cascade laser and the core layer comprising alternatingsemiconductor layers configured to amplify light emitted due to carriertransitions between energy states within the same energy band; andforming one or more p-type electrical isolation regions in a portion ofthe upper n-type cladding layer by adding sufficient p-type dopant todefine a p-type electrical isolation region that extends across at leasta part of the thickness of the upper n-type cladding layer along aprojection separating the laser sections of the quantum cascade laser.20. A method as in claim 19, wherein the upper and lower n-type claddinglayers comprise InP and the p-type dopant is selected such that itsmaximum stable concentration in the InP upper n-type cladding layer isbelow approximately n ×10¹⁸ cm⁻³, where n is less than
 3. 21. A methodas in claim 19 wherein the p-type electrical isolation region is formedby ion implantation.
 22. A method as in claim 19 wherein the p-typeelectrical isolation region is formed by diffusion.